1. FIELD OF THE INVENTION
The invention relates to a method and apparatus for mass spectrometric analysis, and in particular to an apparatus for and a method of spectrometrically analyzing a sample mass that is ionized by a laser beam.
2. DESCRIPTION OF RELATED ART
In one type of conventional mass spectrometric analyzing apparatus, the sample is ionized by an atmospheric ionizing method. For example, the sample is ionized by a glow discharge method. This type of apparatus, however, is restricted in use to ionizing gaseous samples, and is consequently disadvantageous for use in analyzing a wide variety of samples.
An example of a mass spectroscope that uses a laser for ionizing the sample portion is disclosed in the Proceedings of the 23rd Applied Spectrometry in Tokyo, 1988, at 135, 137. In this type of apparatus, a sample is irradiated with a laser beam for ionization, and only the surface of the solid is irradiated. This causes simple ionization of the surface molecules, or generates ions by sputtering. Laser breakdown, which will be described later, is not produced because the power density of the laser beam is low. Therefore, the apparatus is restricted to analyzing the surface of a solid.
In Japanese Patent Publication No. 46340/1983, a method of separating isotopes by irradiating a target with a laser beam for ionization and spectrum analysis of the mass of the ions is disclosed. The object of this method is to separate the isotopes. A laser beam of a very high intensity is used to ionize the target. A plasma is produced, and the ions generated are in a charged state that is greater than ten times as high as the charged state of a single electron. As a result, the same element of the sample in the plasma that is produced has not fewer than ten different charged states. Accordingly, the Z/m (Z being the ion charge, and m the mass) is different for each of the charged states of the same element. If the isotopes are separated by a mass spectrometer, then the same elements are collected by separate depositors (isotope collectors). The target material composition is analyzed with high sensitivity if the same elements are collected by the same depositor of the mass spectrometer. However, in this example, the same elements are collected by separate depositors depending upon the charged states, and different elements having the same Z/m value are collected by the same depositor. As a result, this type of ionizing apparatus is not appropriate for the separation and quantitative determination of only the mass m which is necessary for the analysis of a material composition.
In Japanese Patent Laid-Open No. 78384/1975, a mass spectrometric analysis of particles in an explosive plasma that is produced by laser fusion is disclosed. In this apparatus, the charged particles have the same Z/m value and different initial speeds are introduced to the same detector by utilizing a time-dependence type charged particle separating magnetic field in order to measure the mass and the charge of the particles with high sensitivity. The plasma described in this example is plasma having a high temperature and a high density produced by laser irradiation for nuclear fusion. Since the intensity of the laser beam is high, the ion charges are also high. Accordingly, the same elements have different charged states and this type of ionizing method is unsuitable for the analysis of ordinary material compositions.
In West German Patent Laid-Open No. 252010, a method of spectrometrically analyzing the mass of the ions of a plasma that is produced by a laser deposition apparatus is disclosed. The laser deposition apparatus irradiates the material for the substance to be deposited on a substrate with a laser beam to evaporate the substance in the form of atoms or molecules. Part of the evaporated atoms or molecules are ionized by the irradiation of the laser beam. These ions, atoms or particles ordinarily collide with the ions, atoms or particles therearound and form minute clusters. The clusters having charges or ions are taken out by an electrode and introduced onto the substrate. The clusters or ions adhere to the substrate, thereby forming a thin film. Generally, the evaporated gas contains neutral atoms, particles, and the clusters and ions thereof. In order to observe the mass and the charge of the evaporated substance, therefore, the ion components are introduced to the mass spectrometer so as to spectrometrically analyze them. In the analysis, the evaporated atoms, molecules and ions generated during evaporation and the ion components in the clusters are utilized. This mass spectrometric analysis is different from a mass spectrometric analysis in which a material is positively and efficiently evaporated in the form of atoms and ionized for the purpose of elemental analysis (to determine atomic composition) of the material.
The conventional apparatus, described above, for ionizing samples using a laser beam for various purposes is unsuitable for mass spectrometric analysis intended for the analysis of a material composition. That is, even if a conventional laser apparatus is used in the field of mass spectrometric analysis, the ions generated by the laser beam irradiation are not in a predominantly low charged state, and therefore, are not suitable for mass spectrometric analysis.
In addition, when particle components in a liquid or a solid are analyzed, selective and efficient ionization of the particle components is not taken into adequate consideration in the practice of analysis with conventional apparatus. Therefore, it is difficult to analyze a material of various forms such as solids, liquids, and gases for elemental constituents with high sensitivity.
An analysis apparatus that uses a laser beam for laser breakdown of the sample is known. In such an analyzing method using laser breakdown, fine particles in the liquid are counted by using a sound wave generator, as described in, for example, Japanese Journal of Applied Physics, 1988, 27, at L983. Alternatively, it is known to analyze a liquid for elemental constituents by spectrum analysis of a plasma emission produced by laser breakdown, as described in Applied Spectroscopy, 1984, 38, at 721. That is, mass spectrometric analysis using ions generated by the laser breakdown of a sample is not carried out in these type of apparatus.